"Planks" might be more functional for biking than crunches. (it's using your core to hold you in one position, rather than folding yourself in half repeatedly)

Prone planks are the easiest to describe: get into a push-up position but on elbows and toes instead of hands and toes. Hold it while keeping your body straight until fatigued. Repeat three times.

I can only hold a plank for about 10 seconds. Someday I wanna be able to hold for a minute or more like Trek. Dang, she's strong...